In brief, it's basically a Grand Theft Auto clone. I'm not going to say ripoff, because ripoff would imply that they're making a cheap knockoff to capitalize. In fact, this is a well-made well-realized game. This is not to say that it is not gratuitously violent, sexist, and crude (because it is) but It. Is. Delightful.
I still think GTA San Andreas was the most enjoyable of that series, and it was the most playful and ridiculous. Saints Row seems to take play and ridiculosity as their starting points, and goes far beyond GTA. I basejumped off buildings and beat people to death with a giant purple dildo; I drove a tank straight out of Atari 2600 Combat and drove hookers around on 'dates'. I was embiggened and enlessened by this story.
I can think of no higher encomium than this. I received the Platinum trophy for acquiring all the other trophies in the game. The number of other games that I have meticulously acquired all the trophies for: 0.
This is the game that killed my first PS3, and yet I have no rancor toward it.
I bought the game, literally, on the same day that the developer, THQ, basically announced it was dead. It sounds like its death announcement may have been premature, but still.